As is known, rolling operations lead inevitably to variations appearing to some extent in the magnitudes that are associated with deformation of the metal being rolled. This is a consequence in particular of the fact that the rolling forces and torques, the temperature of the rolled product, and the coefficients of friction do not remain accurately constant during rolling. Inaccuracies due to the way the rolling process is controlled cannot be eliminated completely, and for example there are small variations in instantaneous speeds. There are also disturbances which are due to oscillations caused by imperfections in the drive system of the mill or indeed to wear of the tools used. Variations in the rolling magnitudes and dimensional variations of the product as fed to the mill also contribute to degrading the dimensional attributes of the finished product. As a consequence of all these disturbances, the reference tensions specified by a rolling plan for the various frames of a mill are not complied with. This gives rise to tension or compression stresses being present in those portions of the product that are situated in the intervals between frames.
Tension or compression appear in a product that is engaged in a plurality of successive frames in a continuous run particularly when the product is being inserted into the frames and when the preadjusted speed of each frame is not perfect. If the downstream frame is tending to pull the upstream frame then the product present between the frames will be working in traction; if the upstream frame is tending to push the downstream frame by means of the product, then it is subjected to compression. The difference between the speed Vs.sub.n-1 of a product leaving an upstream frame and its speed Ve.sub.n entering the following frame downstream gives rise to stress .DELTA..tau. which is expressed by Hooke's law, and is as defined below: ##EQU1##
where .DELTA..tau. is the variation in tensile or compressive stress to which the metal is subjected between the two frames, where L is the distance between the frames, and where E is Young's modulus.
When the outlet speed Vs.sub.n-1 of the upstream frame referenced n-1 is not in balance with the inlet speed Ve.sub.n of the following frame referenced n, then the stress in the metal in the interval between the frames modifies and the operating point of each of the two frames shifts towards an equilibrium point where the outlet speed from the upstream frame is equal to the inlet speed of the following frame. As is known, this modification gives rise to modifications in the thickness of the rolled metal and to variations in the slip in the two frames concerned. A phenomenon arises whereby the rolling process is self-stabilizing, but this phenomenon is to the detriment of dimensional tolerances for the product and for the desired profile.
Tensile and compressive forces also appear in a product engaged in a plurality of successive frames during rolling whenever the product is not totally uniform over its entire length and presents variations in section and/or hardness that are associated, for example, with variations in temperature. Thus, variation in the hardness of a product entering a frame n-1 gives rise to variation in its section on leaving said frame and to variation in downstream slip, thus leading to a modification in the rate at which metal is output from the frame.
To remedy those drawbacks, there exist control systems applied to multi-frame mills that include means for monitoring traction in the various intervals between frames by individually regulating the ratio of rolling torque over rolling force on a frame-by-frame basis. Such regulation requires sensors to be present, and in particular rolling force sensors which are expensive, difficult to install and maintain, and which constitute a potential source of breakdowns. In addition, that solution which requires the presence of sensors is not always applicable, particularly in rolling mills for producing bars or girders in which such sensors are rarely installed.